Sarcoma growth factors (SGFs) are peptides capable of interacting with the epidermal growth factor (EGF) receptor system and phenotypically transforming non-transformed cells in tissue culture. These peptides are isolated from serumfree conditioned media of murine sarcoma virus (MSV) transformed cells. This conditioned media contains several peptide factors capable of modifying the growth or differentiation patterns of biological systems. A SGF with an apparent molecular weight of 10,000 has previously been characterized. This is shown not to be a direct product of the sarcoma genome; its expression, however, is controlled by the product of the sarcoma genome. Recently a 20,000 molecular weight peptide has been characterized and purified to apparent homogeneity using exchange and high pressure liquid chromatographies. This peptide competes on a mole for mole basis with EGF for its receptor and is capable of stimulating its phosphorylation. This peptide will be sequenced sufficient quantities have been purified.